[7.17] [Docs] Adds link to default_field index setting to Discover docs (#226629) (#226649)

# Backport

This will backport the following commits from `8.15` to `7.17`:
- [[Docs] Adds link to default_field index setting to Discover docs
(#226629)](#226629)

<!--- Backport version: 10.0.1 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

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Adds link to default_field index setting to Discover docs
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Adds link to default_field index setting to Discover
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Adds link to default_field index setting to Discover docs
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but from a\nfresh PR since the content had moved in that
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BACKPORT-->

Author: florent-leborgne

docs/user/discover.asciidoc

@@ -159,6 +159,7 @@ To search all fields, enter a simple string in the query bar.
 [role="screenshot"]
 image:images/discover-search-field.png[Search field in Discover]
 
+NOTE: Free text searches that don't specify a field may not return expected results depending on how the {ref}/index-modules.html#index-query-default-field[`index.query.default_field` index setting] is configured for the indices matching the current data view.
 
 To search particular fields and
 build more complex queries, use the <<kuery-query,Kibana Query language>>.